Automatic exposure timer for cameras



ec. 11, 1951 R. H. LEE 2,577,774?r AUTOMATIC EXPOSURE TIMER FOR CAMERAS2 Sl-IEETS-SI-!EET 1 Filed Oct. 9, 1946 l FIGURE 3 O (32 r TIMER CAMERA31 35 f3 9" POWER l I FIGURE 4 j SUPPLY FOR TIMER TIMER AND CONTROL 3TIMER cowmgx.

Dec. M W51 R. H. LEE 2,577 774- AUTOMATIC EXPOSURE TIMER FOR CAMERASFiled Oct. 9, 1946 2 SHEETS-SHEET 2 (h Ln \9 K a LL! 0: 9 3 x1 CD a M 9.i n? 5M5.

IIVVE /T O/ P LEE ROBE T i atented Dec. ll, 1951 AUTOMATIC EXPOSURETIMER FOR CAMERAS Robert H. Lee, Indianapolis, Ind., assignor, by

mesne assignments, to Micro Recording Company, a corporation of FloridaApplication October 9, 1946, Serial No. 702,124

6 Claims. 1

My invention relates to improvements in control means for photographiccameras and primarily to means for controlling the timing of theexposure of the object to be photographed to the film or plate of thecamera.

More particularly, my invention relates to means for the timing of theexposure of the object to be photographed to the film or plate of thecamera, in accordance with the lighting of the object or in accordancewith the amount of light reflected by the object to be photographed.

My invention is particularly applicable to the controlling of camerasfor the making of micro.- film reproductions. In the making of microfilmreproductions, due to the fact that the film or plate is so extremelysmall, the negative must be extremely sharp. Where the object to bemicrofilmed is of a characteristically dark texture, and as a result theamount of light reflected by the object into the camera is comparativelyweak, in order to obtain a sharp, clear negative, the period of exposuremust be relatively longer than the period of exposure where thereflected light is considerably greater. Likewise, in the making ofmicrofilm exposure, the object itself is generally illuminated throughthe medium of suitable lamps to direct sufficient illumination on theobject for the purpose of making a clear image, which lamps receivetheir supply current from ordinary electric supply lines. It has beenfound that the voltage of the lines is apt to fluctuate considerably,resulting in a fluctuation of the illumination of the object. Where theillumination drops, the object should be exposed to the negative for alonger period and where the voltage to the lamp increases its lightoutput and therefore the illumination on the object to be protographed,the time of exposure should accordingly be decreased.

My invention therefore lends itself particularly to the control ofmicrofilming cameras and provides a means whereby the period of exposureis determined by the light reflected into the camera from the object tobe photographed.

A further object of my invention is to provide a plurality ofelectromagnetically operated switches which are adapted for successiveoperation and to provide for controlling the successive operation ofsaid switches, a tube for each switch including a cathode, an anode anda grid, said tubes being so energized as to effect the successiveoperation of said switches.

A further object of my invention is the provision of a plurality ofelectromagnetically operated switches adapted for successive operationand the control of said successive operation through the operation of aplurality of tubes each including a cathode, an anode, and a grid, saidtubes. being so arranged that they will provide for the successiveenergization of said electromagnetically operated switches, togetherwith a light responsive means for so controlling the operation of saidtubes as to. control the period between the successive operation of theswitches.

Further objects and advantages of my inven- Will appear more fullyhereinafter in the appended specification and claims.

For the purposev of disclosing the invention, I have illustrated anembodiment thereof, in the accompanying drawings, in which:

Fig. 1 is an end elevation (more or less diagrammatic) of a microfilmcamera and its control mechanism;

Fig. 2 is a longitudinal, section view (more or less diagrammatic) ofthe operating mechanism of the camera;

Fig. 3 is an end elevation of the operating mechanism for the camerashutter;

Fig. 4 is a diagrammatic View showing the association of the variouscontrol mechanisms; and

Fig. 5 is a, diagrammatic view of the circuit arrangement of theoperating mechanism and parts.

In the embodiment illustrated, having particular reference to Figs. 1 to4, I provide a microfilm camera 1 suitably supported above the object 2and alongside the camera, I preferably mount the timing controlmechanism 3 so that the light reflected from the object beneath thecamera to the timing control will be substantially the same as, if notidentical with, that reflected into the lens of the camera. This timingcontrol includes a casing having a small opening in its bottom throughwhich the light reflected from the object is adapted to pass toinfluence the electricv eye forming a part of the timing control. Forthe purpose of providing suitable illumination for the object, Ipreferably provide on each side of the camera illuminating lamps ithaving associated therewith suitable reflectors 5.

The camera I is provided with a suitable, lens 6 through which the raysor light pass to impinge upon a film negative mounted within the camera.The admission of light through the lens and onto the negative iscontrolled by a shutter 1 pivoted to. swing. on a pivot support or shaft8. This shutter L is adapted to be opened and closed through the mediumof suitable electroinagnets 9 and In between which is mounted anarmature carried on a suitable shaft l2, provided with an arm l3 havinga rack l4 meshing witha pinion I5 mounted on the shaft 8 carrying theshutter so that as the shaft is rotated in one direction or the other,the shutter Will be opened or closed.

The negative film is adapted to be wound from a supply spool |6 onto areceiving spool These respective spools are driven by suitablebelt'drives l8 and I9 from a pulley of an electric motor 2|. The circuitof this electric motor is controlled by a switch including a switch arm22 and a relatively stationary contact 23. The switch arm 22 is biasedin an open position by suitable coil spring 24 and is carried by an arm25 pivoted at 26. This arm 25 is moved in one direction against the biasof the spring 24 through the medium of an electromagnet 21. The lowerend of the arm is adapted to ride on a cam 28 carried on the shaft ofthe reel ll, which cam has a low spot into which the end of the armgram, the power supply unit 29 has a cable 36 connected with a suitablesource of alternating current supply. From this power supply unit, a

cable 3| extends to the timer unit 32 and likewise a cable 33extendsfrom the power supply unit to the timer control unit 34. There is also acable connection 35 between the timer and the timer control unit with acable connection 36 extending between the timer and the operatingmechanism of the camera In Fig. 5, I have illustrated a diagrammaticview of a circuit arrangement of the various control elements and theircircuit relation with one another, for controlling the operation of thedifferent components of the camera. In the circuit arrangement shown,the two leads 31 and 38 which are adapted to be connected with anysuitable alternating current supply, in the specific case a 115 voltalternating current supply, are

connected to the opposite terminals of the primary winding 39 of atransformer 40 through suitable fuses and through a suitable manuallyoperated control switch 4|.

A secondary winding 42 of the transformer supplies filament or heaterpower to rectifyingtubes 43 and 44 as well as to the gas discharge tubes45, 46 and 41, by

conductors 48 and 49. A winding 50 of the transformer 46 suppliesalternating current voltage to the rectifying tubes 43 and 44 which arearranged in a voltage doubler circuit and furnish,

in the embodiment illustrated, 150 volt direct our .rent plate powerthrough conductors 5| and 52 to the gas triodes or tubes 45, 46 and 41and furnishes negative bias (150 volt direct current) through conductors52' and 48' for control of the grids on the triode tubes 45, 46 and 41.A winding 53 of the transformer 4|] supplies filament power to arectifier tube 54 and a winding 55 of the transformer 40 suppliesalternating current voltage to the rectifier tube 54, which suppliesthrough conductor 55 direct current plate power to a tube 56 which isconnected in series with a tube 56'. The current return is through theconductor 51. The condensers 58', 59' and 60' are for the purpose offiltering the three direct current output voltages supplied to the timerrcircuits (the timer circuits will be more fully described hereinafter),the condenser 58' being connected across the conductors 48 and 5| andthe condenser 59' being connected across the conductors 48' and 52 whilethe condenser 66 is connected across the conductors 55 and 51.

Action is initiated through the momentary closure of the manuallyoperated switch 58 which completes the circuit from the lead orconductor 59 through the coil 60 of a relay controlling switches 6| and62, through switch 63 back to lead 64. Hereafter reference will be madeto the switches controlled by the relays having coils 60, 69 and 89. Oneposition of these switches will be referred to as the operated positionand the other position of the switches will be referred to as thereleased position. The term operated position is used to indicate theposition of the switches when they are moved from the positionillustrated in the drawings downwardly, under the influence of therelays having the coils 66, 69 and 89 and the term released position isused to indicate their normal or biased position or the positionillustrated in drawings.

With the switch 62 in closed position, the circuit from the alternatingcurrent power source to the solenoid I0 is completed by way of conductor64 through the switches 62 and 63 and by the return conductor 66. At thesame instant, switch 6| also closes the circuit between the positivelead 5| volt direct current with respect to conductor 48) through the resister 67 to furnish positive bias on the grid 68 of the tube 45. Thetube 45 passes current from its plate through the coil 69 of theeleotromagnet controlling the relay switches Ti] and H to effectoperation of these switches. The operation of the relay switch 7|initiates no action, since it is in series with the lower contact ofswitch 63 which has not yet been closed. Switch 16 in moving from itsnormal position disconnects the negative lead 52 (-150 volt directcurrent with respect to conductor 48') from the grid 12 of the gastriode tube 46, the grid I2 of this gas triode being connected through acondenser 13 to the negative bias lead 52.

The timer control amplifier circuit including tubes 56 and 56 suppliescurrent to a condenser 13 allowing a charge to build up in thiscondenser '|3. When this charge has become sufficiently built up,through its control of the gas triode tube 46 it effects the closure ofthe camera shutter 7, thus shutting off the exposure of the negative.Accordingly, the time it takes for the charge in the condenser 13determines the length of time of the camera exposure and this build-uptime is controlled by an electric eye tube 14 which is subjected to thelight reflected from the images and is therefore responsive to thevariations in said light. As has heretofore been explained, the timer isarranged in the timer casing 3 disposed beside the camera The electriceye 14 and its associated timing elements is mounted in the casing 3with the electric eye in such a position relative to a small opening inthe bottom of the casing as to receive the light reflected from theobject to be photographed, in the same mamier as the camera lensreceives the light.

-This is more particularly illustrated in Fig. l.

The photoelectric eye 74 is connected in an amplifier circuit whichamplifies the current in the eye, resulting from the light falling onthe screen of the eye, to a value sufficiently high to operate the timermechanism and control the build-up of the charge in the condenser 73.

In this timer control circuit, two voltage conr trol or regulator tubesand 76' are connected in series with a resistor ll across the conductors55 and 51, and supply constant voltage to the amplifying circuit. Thephotoelectric tube 14 is connected between the positive terminal 18 ofthe regulator tube 76 and the control grid 19 of the amplifier tube 59'.The control grid 19 of this amplifier tube 56' is connected to conductor'51 through a resistor to of a value sufficient to create suitablevariations in the voltage of the grid, resulting from current fiow fromthe photoelectric tube M. The cathode 3! or the amplifier tube 59' isconnected to conductor 5'! through a resistor 32, which limits thecurrent flow through the amplifier tube. The plate 83 of the amplifiertube 59' is connected to the cathode 84 of the amplifier tube 55. Aresistor 85 is connected between the positive terminal 78 of theregulator tube 76 and the cathode 8| of the amplifier tube 56' toprovide a bias of such value that the current passing through theamplifier tube 59, when no light is presented to the photoelectric eye14, it will be nearly zero. The plate 85 of the amplifier tube 56 isconnected to the positive terminal ill of the voltage regulator tube 15.The screen grid 98 of the amplifier tube 55 is connected throughconductor 73a to the negative bias lead 52, which is connected to oneside of the condenser l3. The grid 72 of gas triode as is connected tothe other terminal of condenser 13 and also to positive lead 55' throughconductor 13?).

When the voltage across the condenser E3 has reached a pointsuffici-ently high to start a current flow in the gas triode 46, currentflow will be established through the relay winding 59. With the relayWinding 89 energized, the switch 63 is moved to operated position andwith switch switch H in operated position, a circuit is establishedthrough the electromagnet 9, thus moving the camera shutter to closedposition. The current flow through the tube 49 drops the voltage at theplate 99 of the gas triode 45, due to a condenser Nil which is connectedbetween the plate 92 of the tube 48 and the plate 99 of the tube 45,thus causing the tube 45 to cease conducting current through the winding69, deenergizing this winding and permitting switches 79 and H to moveback to normal or open position. A time delay circuit however, isprovided to allow current to flow through relay coil 69 for a period oftime after current flow ceases through the tube t5 and thus, for a shortperiod of time, maintaining switch ll closed for a sufiicient length oftime after the closure of the switch 63 to complete the movement of theshutter l to its closed position. Thus for a short interval of time theswitch ll controlled by the relay coil 99 and the switch 93 controlledby the relay coil 89 are both in closed position, all-owing current flowfrom the conductor through the solenoid 9,

through the switch 93, through the switch H, through the conductor 6 3as above explained, maintaining the current fiow through the winding 9of the solenoid sufiiciently long to complete the closure of theshutter.

The time delay in deenergization of relay coil 69 is accomplished bymeans of a condenser 93 and a resistor 94 in series, being connectedacross the relay coil 69. After current flow has ceased through gastriode 45, the discharge current from condenser 93 through resistor 94keeps the relay coil 69 energized. The period during which the relaycoil 69 is sufficiently energized to keep switches l9 and H in a closedposition is dependent on the capacity of condenser 93, the resistor asram 94, theresistance of relay coil 89, and the charge which was presenton condenser 93 at the instant current flow through gas triode 45ceased. The said values are pre-set to get the desired amount of delaytime in the releasing of switches 19 and H. This delay time is the timeduring which solenoid 9, controlling the closure of the camera shutter'1, remains energized.

Immediately after the closing of the shutter has taken place, the relaycoil as becomes deenergized, thereby releasing switches 70 and H. Theswitch H completes a circuit from the conductor 64 through the lowercontact of switch 63, through the switch H, through the conductor '19 tothe solenoid 21, which operates the switch 22 to close the circuitthrough the motor 2|, driving the film take-up of the camera and thusmoving the film one step to present a fresh film for photographicpurposes. The motor wind-up is stopped when the cam wheel 23 permits thearm 29 of the motor control switch to drop onto the low side of the camwheel, thus opening the switch 22. The circuit through the solenoid 21is also shut off through the release of the switch 63 which is effectedwhen the relay coil 89 is de-energized.

With the switch 95 released, that is with the coil 89 deenergized andswitch 95 in its up position, a short circuit will be established aroundthe condenser es and accordingly, the condenser will not be receiving acharge. Durin the time, however, when the relay coil 89 is energized,the switch 95 will be moved to its down position and as a result, thecondenser as will be connected across the leads 5! and 52, and a chargewill be built up in the condenser, due to the potential differencebetween leads 5| and 52. The control grid 97 of the tube il is connectedto the condenser 96 and when the charge of the condenser 95 has beenbuilt up to reach a sufi'icient positive value to trigger the controlgrid 9?, current flow will be established through the tube M. Withcurrent flowing through tube 4?, the voltage at the plate 92 of the tube55 is lowered by means of a discharge through the condenser 98 which isconnected between the plate 92 of the tube 48 and the plate'Sl' of thetube :17. As it is desirable to have a certain time delay in thedeenergization of the relay coil 89, in which to permit the motor tocomplete its film takeup action, a resistor [9% is connected in serieswith the condenser 96. This time delay is for the purpose or allowingtime for the switch arm 25 to ride out of the notch in cam 28.

With the camera shutter open, the light refiected from the objectenergizes the photoelectric eye I i, which in accordance with thewellknown characteristic of this type of device, will permit currentflow in proportion to the light impinging upon the screen of the eye andthe current will fiow between terminal E8 or" the voltage control tubeit and the grid id of the amplifier tube This current also passesthrough a resistor 89 and as a result, a voltage is set up across theresistor 89 and imposed upon the grid 19 of the amplifier tube 56, andcurrent will then flow through the amplifier tube. lhe plate 83 of theamplifier tube 56' is connected to the cathode 84 of the amplifier tube56. The high positive voltage present on the screen grid 88 of theamplifier tube 56 attracts the electrons from the cathode ml of theamplifier tube 55 and the electrons are accelerated and pass through theaforesaid screen grid to impinge upon the plate 86 of the amplifier tube59. The plate gives amazes up electrons under the bombardment and theseare attracted back to the screen grid 88 of the tube 56 and pass to thenegative terminal of the condenser 13, building up a voltage across thecondenser 13. When this voltage is built up to the critical point,current flow will start through the gas triode 46 and through the relaycoil 89, thus energizing the relay coil 89 which in turn operates theswitches 63 and 95. Switch 63 allows alternating current power to passfrom the line 88, thus closing the circuit of solenoid 9 through theswitch 1 I, upper contact of through switch 63 to conductor 54. Uponclosing of the shutter, no more light is allowed to strike the negative.

Immediately after the shutter has been closed by the solenoid operation,the current flow through relay coil 69, which has resulted from a chargeon condenser 83, has decreased to the point where switches 10 and 1!controlled by relay coil .69 are allowed to release. Release of switch1! allows current to flow from conductor 66 through coil 21, throughswitch 53, through switch 1|, to conductor 64, thus starting the filmwind-up motor.

Immediately after the solenoid 21 has been energized, the current flowthrough relay coil 89, resulting from a charge on condenser 9| hasdecreased to the point where switches 53 and 55 controlled by relay coil89 are allowed to release. In opening the lower contact of switch 63,current flow is shut oiT from the solenoid coil 21. However, upon thestarting of the motor, the arm 25 is mechanically held in position toclose the switch 22 through the medium of a cam driven with the filmtake-up by the motor. Therefore, the opening of the circuit through thesolenoid 21 does not. open the circuit of the motor until the cam 28 ofthe film take-up spool permits the arm 25 to drop into the lower portionof the cam. When this happens, the switch 22 controlling the. operationof the motor is opened, permitting the motor to come to a rest. Theinertia of the motor causes it to continue to turn for a period of timeafter the current flow through the motor is cut off. This coasting ofthe motor is prevented from turning up more film by the medium of a slipbelt.

To further explain the action of the timer circuit, at the instant thatswitch 95 is caused to open by current fiow through the relay coil 89,the circuit from the negative conductor 52 through switch 55 to the gridof gas triode 41 is opened, thus removing a short circuit from aroundcondenser 96. This allows a charge to build up across condenser 95,resulting from current flow through resistor I50 coming from thepositive conductor When the voltage across the condenser 05 has reachedthe critical point, it allows current flow through the gas triode 41 andthrough the resistor N32. The sudden change in voltage at the plate 99of the gas triode 41 causes a charge of current to flow throughcondenser 98, dropping the voltage at the plate 92 of the gas triode 48,which allows the gas triode 46 to cease current flow. Thus, the relaycoil 89 is deenergized, allowing for releasing of the switches and TI.At this instant, the timer has completed its cycle and is ready for thestarting of another cycle. The grid 68 of the gas triode 45, grid 12 ofgas triode 46 and the grid 91 of gas triode 41 are held at a negativepotential by connection to conductor 52, which prevents start of currentflow through any tube. The gas triode 41 at the end of the cycle 13 anddischarges this condenser. contact 1| de -energizes the shutterclosingsoleis conducting current so that holding the grid at thenegative potential does not effect the current flow in this tube. Thepurpose of gas triode 41, resistor I02 and condenser 98 are purely toquench the current flow in gas triode 46, in order to terminate thecycle. The tube 41 is quenched when tube 45 is re-energized to connectcondensers I0! and 98 in shunt to the tube 41, and the charge oncondenser 98 applies a negative voltage to the plate of tube 41 todeenergize this tube.

In operation, the color of the light sources and correspondingly, thecolor of the light reflected from the object vary under difierentconditions of operation. For instance, a change in voltage in the inputline may afiect the color characteristic or intensity of the lights 4-4.In order to compensate for this change, I provide a means forcontrolling the linearity of the amplifying tubes 56 and 55, whichconsists in properly selecting the characteristic of the tubes andassociating with each of the tubes in the cathode circuit respectivelyresistances 82 and 55. These resistances are so selected as to cut downthe response of the tubes 55 and 55' at low light values which effectthe tube 14. By this arrangement, I am enabled to compensate for thedifference in response to varying color values between the lightresponsive means 1'4 on the one hand and the negative on the other.

In order to obtain the above characteristics, in the specific embodimentof my invention illustrated, the tube 55 comprises a 6AC'1 tube with thescreen grid, the suppressor grid, and the plate tied together to obtainthe characteristic of a sharp, cut ofi triode with the resistance 82having a value of 2,000 ohms and a resistance 82' having the value ofone megohm. Likewise, the tube 56 is a 6AC7 tube with the screen gridand the suppressor grid tied together, and the resistor has the value ofone megohm.

Summarizing, the operation is as follows:

Closing of switch 58 energizes relay coil 60 through upper contacts ofswitch 63, thereby closing contacts 6! and 62.

Contact 62 operates solenoid It) to open the shutter. Contact 6| appliespositive voltage to the grid 68 of tube 45 to energize relay coil 69which operates contacts 10 and 'H.

Opening of contact 10 removes a short-circuit around condenser 13 whichbegins to charge from the photocell circuit in a direction to bias thegrid 12 in a positive direction. Operation of contact H has no efiect.As soon as condenser 13 reaches a predetermined charge, tube 45 operatesto energize relay winding 39 which operates contacts 53 and 95.

Operation of contact 63 first opens the circuit of relay coil 60 torelease contacts SI and 62, and then completes a circuit through itslower contact and through switch 1| to energize solenoid 9 which movesthe shutter to closed position.

Upon firing of tube 46, the low resistance path of this tube connectscondenser E0! in shunt to tube 45 and the charge on this condenserapplies a negative voltage to the plate of tube 45 to de-energize thistube. 1

Relay coil 69 is shunted by a delay circuit to maintain this coilenergized a suiiicient time to allow the shutter to move to completelyclosed position, and then relay contacts 10 and ii are released. 7

Release of contact 10 short-circuits condenser Release of noid 9 andenergizes magnet 21 which closes the circuit of the film feeding motorto start the motor into operation.

Opening of contact 95 at the time of operation of relay 89 removes ashort-circuit from around condenser 96 and this condenser beginscharging through resistance I99. After a predctermined time ofsuificient duration to permit the switch arm 25, Figure 2, to ride outof the slot in the cam 20, condenser 95 becomes charged to a valuesufficient to cause operation of the tube 47, and this tube applies anegative stopping voltage to the anode of tube 46 from the charge oncondenser 93. After a short time determined by the delay circuit inshunt to the coil 39, relay contacts 63 and 95 are released, and releaseof contact 63 opens the circuit of the motor starting coil 21, but themotor switch 22-23 is maintained closed by the switch arm 25 engagingthe cam 28, and the motor continues to operate until the switch arm 25drops into the slot in the cam.

Release of the relay contacts 63 and 95 places the circuit in conditionto start another cycle of operation as described above.

When tube 45 is re-energized, the low resistance path of this tubeconnects condensers l9! and 98 in shunt to tube 47, and the charge oncondenser 98 applies a negative voltage to the plate of tube 41 tode-energize this tube.

I claim as my invention:

1. A system for operating the shutter of a camera comprising, incombination, means for opening said shutter, electrically operatedshutter closing means, first and second electromagnetic relays eachhaving a pair of normally open contacts which close upon energization ofthe relay, an energizing circuit for said shutter closing meansincluding the normally open contacts on both of said relays connected inseries circuit relation, means controlled by operation of said shutteropening means for energizing said first relay, and a time delay devicecontrolled by said first relay upon energization thereof for energizingsaid second relay after a predetermined time delay.

2. A system for operating the shutter of a camera comprising, incombination, means for opening said shutter, electrically operatedshutter closing means, first and second electromagnetic relays eachhaving a pair of normally open contacts which close upon energization ofthe relay, an energizing circuit for said shutter closing meansincluding both pairs of said normally open contacts connected in seriescircuit relation, means operated simultaneously with the opening of saidshutter for energizing said first relay, and a time delay devicecontrolled by said first relay upon energization thereof for energizingsaid second relay after a predetermined time delay.

3. A system according to claim 2 and including means responsive to theenergization of said second relay for de-energizing said first relay,and means embodied in one of said relays for insuring the closing of thenormally open contact of said second relay before the opening of thecontact on said first relay.

4. A system according to claim 3 and including means controlled by saidfirst relay in deenergized position for re-setting said time delaydevice.

5. A system for operating the shutter of a camera comprising, incombination, electrically operated means for opening said shutter,electrically operated means for closing said shutter, first and secondelectromagnetic relays each having a pair of normally open contactswhich close upon energization of the relay and said second relay havinga pair of normally closed contacts, an energizing circuit for saidshutter opening means completed through said normally closed contacts,an energizing circuit for said shutter closing means including bothpairs of said normally open contacts connected in series circuitrelation, control means for energizing said first relay, and a timedelay device controlled by said first relay upon energization thereoffor energizing said second relay after a predetermined time delay.

6. A system according to claim 5 wherein said control means includes acontrol relay having two pairs of normally open contacts, one pair ofsaid normally open contacts being included in the energizing circuit ofsaid shutter opening means, and the other pair of normally open contactson said control relay being connected to control the energization ofsaid first relay.

ROBERT H. LEE.

REFERENCES CITED The following references are of record in the file ofthis patent:

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